The invention relates to a method of destroying harmful microorganisms in liquids by short-time heating of the liquid in a micro heat exchanger for example by ultra high heating of liquid foods such as milk or other liquids in three steps wherein the liquid is first heated to a high temperature then maintained at this temperature for a predetermined period and is then again cooled down.
It has been common practice to destroy harmful microorganisms in liquids, particularly in liquid foods, by a heat treatment such as pasteurization, ultra-pasteurization or similar processes. Such treatment results in an improved quality and in an increased preservation time. Specifically, in these processes, the liquid, for example milk, is first heated and then kept at a relatively high temperature for a certain time and is then cooled. For this purpose, steam injection is used, but also conventional tube or plate heat exchangers are used, which results in very different residence times for fractions of the products in the heat exchanger. There may be low flow speeds in the heat exchanger particularly in the vicinity of the walls so that the product may be subject to heat damages. The heating times and cooling times as well as the temperatures achieved will be discussed later on the basis of FIG. 2. In laboratory research, capillary tubes have also already been used for the heating of the liquid which capillary tubes had a diameter of 1 to 3 mm. However, for heating relatively large amounts of liquid in an industrial process, such capillary tubes are not suitable.
EP-A-0722075 discloses a tube heat exchanger, which is used for the sterilization of liquids contaminated with microorganisms. In this heat exchanger all tubes through which the liquid is conducted as well as the supply and the discharge passages in the heat exchanger have an identical length and an identical cross-section such that the residence times of the liquid in the tubes of the heat exchanger remains in narrow limits. The tubes are heated by a liquid, by steam or a gaseous heating medium, which is conducted through the heat exchanger by way of two nozzles. By a particular design of the cross-sections, the dimensions, and the arrangement of the tubes in the housing, the efficiency of the tube heat exchanger can be substantially increased so that the liquid can be heated in the tubes to 140xc2x0 C. within a few tenth of a second. With several such tube heat exchangers arranged in series, a three stage heat treatment of liquids with a heating stage, a residence stage and a cooling stage can be provided. In order to further increase the efficiency, it is recommended to use the individual heat exchangers without intermediate supply and discharge channels. It is also pointed out that a number of heat exchangers could be used which are arranged in parallel flow circuits for increasing the flow volume of the liquid.
It is the object of the present invention to provide a method for destroying harmful microorganisms in liquids particularly liquid foods, such as milk, using a heat exchanger arrangement which, by its design, permits rapid heating of the liquid without steam injection and again rapid cooling of the liquid so that the liquid can be maintained at a temperature of over 140xc2x0 C. for only a predetermined short period so as to preserve the food values contained in the liquid.
In a method of destroying harmful microorganisms in liquids by short term high-temperature heating of the liquids in micro heat exchangers formed by stacks of metal foils having parallel microchannels, in a first step, the liquid is rapidly heated in a micro heat exchanger with a gradient of at least 200xc2x0 K/sec, then is maintained at the high temperature for a time period of less than 2 sec and is finally cooled down, again in a micro heat exchanger, with a gradient of at least 200xc2x0 K/sec.